Is the User Name the full email address, or did you just delete it for privacy purposes on this forum? Check there are no obsolete passwords in Options/Security/Passwords/Saved Passwords. Disable the AVG Mail Shield or any other option that may affect TB.

Thanks for the response. The username was deleted because for privacy. Password was reset and shows up correctly in Tools/Options/Security, added Thunderbird.exe to Windows 10 MS Firewall to prevent app blocking, disabled AVG antivirus, etc. Still not able to send email....

To rule out AVG as the source of the issue, I recommend you run in Windows safe mode with networking and see if the problem remains, even though it doesn't seem to affect the Yahoo account.

A regular hotmail or outlook.com account works fine with the same server settings, so I suspect there's some additional configuration required with godaddy to make it work with a hosted O365 account.

Here's the latest - I deleted the account from Thunderbird. I went to File>New>Existing Email. The original setup was for IMAP, but the automatic setup changed to Exchange Server. Besides my email and password, I had the enter the website \xxx .org\username. It seemed to take it. It seemed to install correctly. The bad news is - it setup two-step verification, so I receive a text message code every time I want to log in. I can send SOME email - worked for email on another Godaddy site, but not Yahoo email. I'm the email admin for the website, and the yahoo email I'm trying to send to is the recovery email for the website email account I'm trying to setup. So.... Any ideas on how to turn off the two-step verification and let me send email to a Yahoo email account?

edited link

Can't say why sending to a Yahoo account fails, but I believe two-step verification, if you can't disable it, requires you apply an app password for mail clients in place of the regular account password.